1. Field of the Invention
The present invention relates to tissue repair, and in particular multi-layered matrix combined with cell blocks for tissue repair.
2. Description of the Related Art
Articular cartilage formed on the articular extremities, or surface of bones is a multi-functional tissue and due to elastic property, can break the force of concussions, lubricate the surface of bones with low friction coefficient, and enable perfect ease and freedom of movement between the bones. Cartilage cells, chondrocytes, are about 2% by weight of the articular cartilage and covered with plenty of extracellular matrices. The major difference of articular cartilage to other tissues is that it does not contain blood-vessels, lymphatic vessels, or nerves. Cartilage metabolism is relatively slow in comparison with other tissues; hence, it is much more difficult for defects in cartilage to heal spontaneously. Patients with articular cartilage defects may not feel pain since no nerve is distributed in the articular cartilage. Chondrocytes covered by cell matrices are well-differentiated cells and have low division ability. In addition, mesenchymal stem cells will not be evoked and migrated to the injured area since cartilage lacks blood vessels and lymphatic vessels.
Articular cartilage defects can be classified into partial thickness defect and full thickness defect according to their severity. Partial-thickness defect is a lesion or erosion on the cartilage tissue of the articular surface that does not reach the subchondral bone whereas full-thickness defect penetrates the subchondral bone. With the advances in surgery and arthroscopy, partial thickness defects may be treated or its symptoms may be relieved by surgery or arthroscopic methods such as abrasion arthroplasty, debridement and lavage, high tibial osteotomy, however, these surgeries cannot treat severe damage such as full thickness defects. As a result, patients are faced with the only choice of undergoing both joint excision and replacement with an artificial joint to relieve the pain and regain joint function. In the United States, it is estimated that over 150,000 knee replacement operations caused by full thickness defects annually and the number of such operations is increasing year by year. Artificial joints are expensive as is replacement operation. In addition, artificial joints made of metal only have a ten- to twenty-year life-span. For young patients, a second replacement operation is inevitable, however, older patients may not be able to receive a second replacement operation and become disabled at the rest of their life. Development of a new treatment for full thickness defects of cartilage is therefore very important.
Methods available to treat cartilage full thickness defects include microfracturing and drilling. This technology is a marrow stimulating arthroscopic procedure to penetrate the subchondral bone to induce fibrin clot formation and the migration of primitive stem cells from the bone marrow into the defective cartilage location. More particularly, the base of the defective area is shaved or scraped to induce bleeding. An arthroscopic awl or pick is then used to make small holes or microfractures in the subchondral bone plate. The end of the awl is manually struck with a mallet to form the holes while care is made not to penetrate too deeply and damage the subchondral plate. The holes penetrate a vascularisation zone and stimulate the formation of a fibrin clot containing pluripotential stem cells. The clot fills the defect and matures into fibrocartilage. Microfracturing the subchondral bone plate can be a successful procedure for producing fibrocartilaginous tissue and repairing defective articular cartilage, however, it still has some disadvantages. For example, the microfractures or holes are manually created. If the holes are not deep enough, then the formation of the fibrin clot may not occur. On the other hand, if the holes are too deep, the subchondral bone plate can be damaged and lead to unwanted consequences and complications. In addition, the fibrocartilage formation may fill the defects, but the cartilage function cannot be totally restored. Another technology is Mosaic Plasty procedure developed by a Hungarian surgeon in 1995. This technique involves using a series of dowel cutting instruments to harvest a plug of articular cartilage and subchondral bone from a donor site, which can then be implanted into a core made into the defect site. By repeating this process, transferring a series of plugs, and by placing them in close proximity to one another, in mosaic-like fashion, a new grafted hyaline cartilage surface can be established. The result is a hyaline-like surface interposed with a fibrocartilage healing response between each graft. The advantages of this technique include the grafts are the patient's own tissue and allograft or xenograft rejection can be prevented. In addition, the grafts are biphasic joint containing cartilage and bone and can be implanted to the articular surface to provide excellent support while the surrounding bone tissue grows into the bone portion of the grafts. This procedure, however, is technically difficult. In addition, the grafts are obtained from the unstressed area of the patient, which is limited to a restrained area. The grafting may also destroy the integrity of the joint.
Recently, a new approach for restoration of articular cartilage defects by ex vivo multiplied autologous cartilage has been developed. Chondrocytes from healthy articular cartilage are harvested and the extracellular matrices are digested by enzymes. Chondrocytes are multiplied outside the body for 11 to 21 days to be more than ten times the original number. The cell concentration is adjusted to 2.6×106-5×106 cells/ml, and the cells are then injected into the defect site covered with a layer of periosteum by suturing prior to the injection. This technique is under clinical trial, however, and faces a problem in that chondrocytes are dedifferentiated during the ex vivo multiplication step. The originally rounded chondrocytes become spindle-shaped fibrocartilages and the biochemical properties of the cells are also altered. In addition, the steps of obtaining and suturing autologous periosteum cannot be performed under endoscope. Moreover, it requires at least two surgical procedures (i.e., one to harvest the cells and one to reimplant them); it is relatively expensive; and there are limits in the size of lesion, and the number of lesions, that can be treated.
Other techniques combine materials and cells to repair full thickness defects in cartilage or bone. Biomedical materials are selected based on the physical and mechanical properties of cartilage or bone. For cartilage, naturally occurring or synthetic bioabsorbable polymeric materials are selected, such as collagen, gelatin, alginate, poly (glycolide), poly (lactide) (PLLA), poly (glycolide co-lactide) (PLGA). For bone, biomedical ceramic materials are selected, such as hydroxyapatite, tricalcium phosphate, calcium carbonate, or calcium sulfate. The combination of bioabsorbable polymeric materials and biomedical ceramic materials to mimic bones has also been proposed. As for the structure of cartilage, porous structure is prepared to introduce surrounding tissues thereto or as a scaffold for the implanted cells. In addition, the combination of chondrocytes and gel to from a hydrogel with cells has been proposed. The hydrogel with cells can be attached on the bone-layer material to from a biphasic structure of bone and cartilage. The bone-layer material is also porous to introduce surrounding bone tissue thereto since bone tissue has a stronger regeneration ability than cartilage. As for the combination of materials and cells, small amount of autologous cartilage tissue is harvested, digested with enzymes to remove extracellular matrices and release chondrocytes, and the chondrocytes are implanted into a porous scaffold for multiplication. An appropriate amount of multiplied chondrocytes are then implanted to the defect site. In general, this technique is used for simple evenly-distributed tissue, not for multi-layered tissue. When two different cells are implanted in a porous matrix, cells may flow and mix since the cell size is smaller than the pore size of the matrix. The recent technique for multi-layer cultivation involves ex vivo multiplication of cartilage and bone tissues separately, implantation of the multiplied cartilage and bone tissues to two different porous matrices respectively, combination of the two matrices containing cartilage and bone tissues, and fusion of the borders of the two matrices by refusion cultivation to form a biphasic matrix. This technique is, however, time-consuming and also not clinically applied yet.
It therefore would be advantageous to provide a more effective method of tissue repair using a multi-layered matrix.